Evading authenticated diffie hellman with MITM

Evading authenticated diffie hellman with MITM

I understand that in a non-authenticated Diffie-Hellman setup, a man-in-the-middle attack can occur. Now i'm curious about the feasibility of the following scenario:

Let's assume a situation where www.example.com exclusively supports authenticated Diffie-Hellman.

1. The victim attempts to establish contact with https://www.example.com/.
2. A man-in-the-middle attacker intervenes, retrieves the certificate from example.com, and sends it back to the victim.
3. The victim successfully authenticates the server because the certificate is valid
4. The attacker proposes only unauthenticated Diffie-Hellman to the victim as method of encryption.
5. The victim agrees to the proposal, resulting in an SSL connection being established between the victim and the attacker.
6. The attacker initiates an SSL connection with example.com, which is authenticated due to the initial assumption.
7. Requests made by the victim traverse these two connections.

In this manner, the man-in-the-middle attacker maintains two connections simultaneously.